The present invention relates generally to sensor technology. More particularly, the present invention relates to sensing devices and systems used to monitor large structures using wireless communication techniques.
Many large structures are prone to degradation over time. For example, a large structure including metal elements, such as an aircraft or concrete structure with metal reinforcement, is often susceptible to metal corrosion. Active monitoring and maintenance of a structure may alleviate degradation. Conventional sensing techniques frequently do not provide suitable monitoring solutions for monitoring the health of a large structure. For a concrete structure with metal reinforcement, the metal is often embedded within the structure and inaccessible using sensors that employ a wire for external communication. In some applications, the structure may be so large that hundreds or thousands of sensors are needed for health observation and information collection. For structures comprising concrete for example, concrete strength and lifetime performance are strongly dependent on curing conditions. Construction personnel would ideally like to monitor the temperature of discreet points within the concrete during the curing process. This is not permissible according to conventional techniques.
Inspection of aging bridge decks, as another example, is an important component of an effective highway maintenance program. Pavement and concrete exposed to heavy traffic and weathering deteriorate over time. There are an abundant number of bridges (about 600,000) in United States—and California alone has over 12,000 bridges. Each bridge is inspected on a regular basis. Most of the bridges include metal elements such as tensile bars and metal grids embedded in the concrete. Chloride from the environment surrounding the bridge diffuses into the concrete at its surface, and diffuses from the surface into the concrete depths. The chloride ions may originate from deicing salts in cold climates or salt water from seawater spray in coastal zones, or runoff from nearby (salty) soil embankments, for example. Over time, the amount of chloride in the concrete increases and may reach levels that substantially attack and corrode the metal elements. For a given metal, there is typically a chloride concentration level where corrosion initiates. Steel rebar for example will corrode in the presence of chloride ions at known critical levels. The resulting corrosion leads to expansion of the rebar volume and cracking of the concrete. Monitoring corrosion of the metal caused by chloride ingress is a major objective of regular highway maintenance inspections. Since the metal bars and grids are usually embedded within the concrete at a particular depth, chloride levels at varying depths of the concrete may be monitored to track chloride ion ingress and to detect when chloride presence in the concrete is approaching levels of interest.
Current inspection techniques include visual observation and manual extraction of core samples. Highway engineers frequently take core samples and submit them for laboratory analysis to determine chloride penetration. This process typically involves removing a cylindrical plug from the concrete. Highway engineers then send the plug to a lab and wait for results. Laboratory analysis involves slicing the sample into layers, crushing individual layers, dissolving the layers into solution, followed by a multi-step titration process to determine the chloride concentration. In addition to performing this analysis, individual samples need to be labeled and tracked to correlate lab measurements with specific bridges.
Not only highly time-consuming, costly, and prone to confusion, this method also aggravates the same problem it intends to detect and prevent. Namely, highway engineers refill the hole with a plug. Given the inevitable mismatch of plug materials and sizing, chloride ions now have an easier route into the concrete depths. These manual techniques may also contribute to traffic congestion. Moreover, it typically takes years for a critical chloride concentration to be reached, so a large number of these inefficient manual tests may be required.
In view of the foregoing, there are desired improved structures and techniques for monitoring the health of large structures, such as bridge decks.